Fix TestImageService's unit tests

The test class had hardcoded DEFAULT_IMAGE_DIR value to /img/
which resulted in unit tests failures when this path existed
because the tests expect a call to makedirs which didn't happen
as the DEFAULT_IMAGE_DIR already existed.
The patch fixes that by passing exist_ok=True to makedirs call
which will result in always calling makedirs, whether img_dir
exists or not.

Change-Id: I47d15747536394006f6b502b3025220a5077e547

config_tempest/services/image.py

@@ -128,11 +128,8 @@ class ImageService(VersionedService):
         name = image_path[image_path.rfind('/') + 1:]
         if self.convert and name[-4:] == ".img":
             name = name[:-4] + ".raw"
-        if not os.path.exists(img_dir):
-            try:
-                os.makedirs(img_dir)
-            except OSError:
-                raise
+        # create img_dir if it doesn't exist already
+        os.makedirs(img_dir, exist_ok=True)
         alt_name = name + "_alt"
         image_id = None
         if conf.has_option('compute', 'image_ref'):